The invention relates to a method for the manufacture of a coating and also to a body coated with this method.
Large areas can be uniformly provided with thin layers using a special LPPS process (“Low pressure plasma spraying process”), namely using a LPPS-TF process (“LPPS—thin film process”) which is known from U.S. Pat. No. 5,853,815. This process is a thermal low pressure spraying method with which layer thicknesses in the micrometer range can be produced. The manufacture of a uniform coating is attained by means of a spraying gun, which is geometrically suitably designed, with it also being essential that a sufficiently large overpressure prevails in the spraying gun. The surface to be coated is brought into a process chamber, in which a pressure lower than 10 kPa is generated, while in the spray gun for example there is a pressure of approximately 100 kPa, in other words approximately environmental pressure prevails. The large pressure difference between the inside of the spray gun and the process chamber has the effect that the thermal process beam expands to a broad beam in which the material to be sprayed is uniformly distributed. A thin layer is deposited which is dense due to the homogeneous process beam. A thicker coating with special characteristics can be intentionally produced by means of multiple application of layers of this kind.
A coating of this kind can be used as a functional layer. A functional layer can be applied to a metallic base body, which forms the substrate to be coated, and is as a rule a multiple layer system with differently composed part layers. For example, the blades for gas turbines (stationary gas turbines or aircraft engines) which are operated at high process temperatures are coated with a first part layer comprising a single layer or a multiple layer, so that the substrate becomes resistant to hot gas corrosion. A second part layer—of ceramic material—forms a heat insulating layer. A method for the production of such a heat insulating layer system is described in EP-A-1 260 602, with which a plurality of layers (barrier layer, protection layer, heat insulating layer, smoothing layer) are applied in one working cycle by means of a changing setting of controllable process parameters.
In a special LPPS-TF process, a hybrid coating is carried out with the thermal process beam. This process, which is known from EP-A-1 034 843 or EP-A-1 479 788, permits the combination of thermal spraying with a vapour phase deposition and thus possibilities of combining the two methods. The characteristics of the process beam are determined by controllable process parameters, in particular by the parameters pressure, enthalpy, composition of a process gas mixture and composition and also the application form of the material to be sprayed. The coating material is partly vaporised in dependence on the controllable parameters. The phases of the coating material present in vapour form and condensed form, i.e. in solid or fluid form, are at least partly deposited on the substrate. The relative proportion of vapour and/or condensed phase for the material transported in the process beam is determined by means of a diagnostic measuring method. The controllable process parameters are adjusted in relation to desired values using measuring data gained in this way. With regard to these desired values, which correspond to a specified proportion of vapour or proportion of the condensed phase, a regulation of the intentional manufacture of the multiple layer system is carried out.
Using the hybrid coating method a heat insulating layer can be manufactured with a columnar microstructure. This deposit or layer is composed approximately of cylindrical particles or corpuscles, the central axes of which are directed perpendicular to the substrate surface. Transition regions, in which the density of the deposited material is smaller than in the corpuscles, bound the corpuscles at the side. This columnar layer with anisotropic microstructure is tolerant to elongation with respect to changing stresses, which result due to repeatedly occurring temperature changes. The coating reacts to the changing stresses in a largely reversible manner, i.e. without a formation of cracks, so that its natural life is increased considerably in comparison with the natural life of a coating which has no columnar microstructure.